Electrochemical measurements are widely used to determine the concentration of specific substances in fluids. These devices, referred to as ion-selective electrodes (ISEs), can be employed in a wide variety of potentiometric ion determinations, including, for example, the activity of fluoride ion in drinking water, the pH of process streams, and the determination of electrolytes in blood serum.
In the health care field, and, in particularly, in the area of clinical diagnostics, ISEs are commonly used to measure the activity or concentration of various ions and metabolites present in blood, plasma or serum, urine, and other biological fluids. For example, ISEs are typically used to determine Na.sup.+, Ca.sup.++, Mg.sup.++, K.sup.+, Cl.sup.-, Li.sup.+ ions as well as the pH, and carbon dioxide content in such fluids.
Conventional ion-selective electrodes are typically composed of an ion-selective membrane, an internal filling solution or electrolyte, and an internal reference electrode. Ion-selective electrodes can be classified according to the nature of the membrane material, and include solid state membrane electrodes, glass membrane electrodes, liquid membrane electrodes having charged ion-selective agents, and neutral liquid membrane electrodes having membranes formed from an organic solution containing an electrically neutral, ion-selective agent such as an ionophore held in an inert polymer matrix. An external reference electrode used in conjunction with the ISE to effect the assay measurement is typically a metal/metal halide electrode such as Ag/AgCl.
When the ion selective membrane electrode is exposed or subjected to a sample solution, the ion of interest is selectively transferred from the sample solution into the membrane. The charge associated with the ions generate a potential that can be mathematically related to the concentration or activity of the ion content in the sample. If the membrane is ion specific or ideally selective toward the ion of interest, the potential difference is a linear function of the logarithm of the activity ratio of the ion activity (Nernst equation). A semi-empirical extension of the Nernst Equation (Nikolskii Eisenmann equation) for EMF may be utilized for non-ideal conditions. By "EMF" is meant the electrical potential difference between the internal ion sensing electrode and external reference electrode, the electrodes being electrolytically connected by means of the sample solution at zero or near zero current flow.
Conventional ISEs are typically bulky, expensive, difficult to clean and maintain, and tend to require an undesirably large volume of biological fluid. For these reasons, much attention has been directed towards developing more reliable ISEs of smaller size. These relatively small ISEs, referred to as ion-selective sensors or biosensors, can be inexpensively mass produced using techniques similar to those employed in the manufacture of electronic components, including, for example, photolithography, screen printing, and ion-implantation.
Ion-selective sensors and biosensors can be manufactured at much lower production cost than conventional ISEs, making it economically feasible to offer a single-use or limited-use disposable device, thereby eliminating the difficulty of cleaning and maintaining conventional ISEs. The reduced size of ion-selective sensors further serve to reduce the required volume of patient sample. Generally, a sensor can be either a miniature version of a conventional electrode or a device constructed using one or more of the above mentioned techniques. Maximum accuracy of the analytical or diagnostic result is obtained when the sensor responds only to the concentration or activity of the component of interest and has a response independent of the presence of interfering ions and/or underlying membrane matrix effects. The desired selectivity is often achieved by an ion-selective membrane containing an ion-selective agent such as an ionophore positioned over an electrical conductor.
Generally, ion-selective membranes are formed from a plasticized polymer matrix, such as polyvinyl chloride, which contains the ionophore selective for the ion of interest. For example, natural polyether antibiotic monensin is a known ionophore which is selective for sodium (Na.sup.+) ions (Lutz, W. K., et al., Helv Chim ACTA, 53, 1741 (1970).
Monensin ester derivatives such as methyl, ethyl, and butyl monensin esters have also been used as ionophores or sodium-selective agents in membranes for ISE applications. For example, highly lipophilic monensin derivatives have been reported in ISE applications for serum Na.sup.+ determinations (Tohda, K. et al. Analytical Sciences, vol. 6 (April 1990).
One known limitation of monensin ester derivatives is their low selectivity for Na.sup.+ relative to potassium ions (K.sup.+). This is not particularly troublesome in human serum samples where a selectivity factor of K.sub.Na/K =0.5 for sodium relative to potassium ion results in an insignificant potassium error due to the presence in human serum of a maximum potassium concentration range of 2 to 8 mEq/L and a nominal sodium concentration range of 145 mEq/L. In contrast, urine samples can contain potassium concentrations ranging as low as 1 mEq/L or less up to 100 mEq/L along with relatively low sodium levels of 10 mEq/L. Thus, accurate determinations of sodium ions in urine samples can be troublesome when made using known monensin ester derivatives having low selectivity for Na.sup.+ relative to potassium ions (K.sup.+). This problem is typically handled by performing parallel potassium ion determinations and correcting any inaccurate sodium determinations.
Other known synthetic ionophores or ion-selective agents for sodium include polyether diamide, biscrown ether (Tamura et al., Anal. Chem., 64, 2508 (1992)), cryptand (Lehn et al., Science, vol. 227, no. 4689, p.849 (1985)), and calixarene (Kimura et al., Anal. Chem., 64, p. 2508 (1992)). However, the effectiveness of these sodium ionophores can be affected by interfering substances including drugs and interfering cations present in biological samples, such as urine.
Maryuma, K. et al., Enantiomer Recognition of Organic Ammonium Salts by Podand and Crown Type Monensin Amides: New Synthetic Strategy for Chiral Receptors, Tetrahedron, vol. 48., no. 5, pp. 805-818 (1992) disclose monensin amide derivatives exhibiting the same Na.sup.+ selectivity as biological monensin and its ester. Maryuma, K. et al. do not mention the monensin derivatives of the present invention. Maryuma, K. et al., New Chiral Host Molecules Derived From Naturally Occurring Monensin Ionophore, J. Chem. Soc., Commun., p. 864 (1989) disclose monensin derivatives having neutral terminal groups which show enantiomeric selectivity for several amine salts, whereas natural monensin could not discriminate between the enantiomers. Neither of the Maryuma, K. et al. references mention or suggest the monensin amide derivatives of the present invention such as the hexadecyl monensin amide or methyl hexyl monensin amide sodium-selective agents.
There is a need for a sodium-selective agent for use in an ISE which provides for adequate sodium selectivity and which minimizes or reduces the effects of interfering substances such as drugs and interfering cations present in biological samples such as urine samples.